Conventionally, there is known a wiring substrate that includes a through-wiring formed by filling a through-hole of a core layer with plating. Such wiring substrate may be manufactured by the following method (see, for example, Japanese Laid-Open Patent Publication No. 2003-46248). First, a resin plate, which is to become a core layer, is prepared. Both surfaces of the resin plate have copper foil formed thereon. Then, a hole is formed in both surfaces of the resin plate by etching the copper foil formed on both surfaces of the resin plate. Then, a through-hole is formed by performing a laser process on each of the holes formed in the surfaces of the resin plate. The through-hole has a shape in which two tapered peak parts confront each other.
Then, the through-hole is filled with plating. More specifically, at an early stage of filling the through-hole with plating, two closed-end via holes are formed by closing up a narrowest part of the through-hole. Then, unpatterned plating layers are uniformly (entirely) formed on the copper foils of both surfaces of the resin plate in addition to filling the two closed end via holes with plating (i.e. filling the entire through-hole with plating).
However, by using the above-described method, the plating layers that are uniformly formed on both surfaces of the resin plate become inevitably thick. Accordingly, in a case of forming patterns (patterning) on the copper foils or the uniformly formed plating layer by using a subtractive method, it becomes difficult to form the patterns with a fine pitch. For example, if the thickness of the copper foils are approximately 12-18 μm, the total thickness of the copper foil and the plating layer on each surface of the resin plate would become several tens of micrometers (μm). Thus, it is difficult to form fine-pitched patterns on the copper foil and the uniformly formed plating layer by using the subtractive method.